1. Field of the Invention
The invention pertains to the field of encoding and decoding of optical signals and to the field of semiconductor optoelectronic devices.
2. Description of Related Art
The rate of information transfer increases many-fold each decade. A permanent need in a speed increase and reducing power consumption leads to the fact that the application field of optoelectronic devices used in the information transfer is gradually expanding. FIG. 1 shows schematically a general scheme of the information transfer using optoelectronic components. An electric circuit generates an electric input signal which comes to a transmitter. The transmitter transforms an electric signal into an optical input signal. The optical input signal propagates through an optical medium and comes to a receiver in a form of an optical output signal. If the optical medium has no or a low dispersion, the optical output signal repeats the shape of the optical input signal. The receiver transforms an optical output signal into an electrical output signal.
Typically, to ensure robust and error-free operation, a digital encoding of the signal is used. For example, a sequence of pulses using a two-level encoding is applied, i.e. the pulse amplitudes alternate between the level “zero” and the level “one”. FIG. 2(a) shows schematically a sequence of input electric pulses applied using the two-level encoding systems. In an idealized case, the sequence of output electric pulses should be the same, as shown in FIG. 2(b).
The repetition rate of pulses is usually limited by the high frequency performance of the transmitter and receiver. To increase the information transfer rate keeping the same repetition rate of pulses, a multi-level encoding can be used. FIG. 2(c) shows schematically a sequence of input electric pulses in a scheme using three-level encoding. In an idealized case, the sequence of output electric pulses should be the same, as shown in FIG. 2(d).
However, multilevel encoding faces severe problems regarding generating multi-level optical pulses by a transmitter. Operation of an optoelectronic device is sensitive to temperature variations, and the level of an optical signal generated by the transmitter can deviate from the pre-set value. FIG. 3 illustrates such a deviation. FIG. 3(a) shows a sequence of pulses of the electric input signal, and FIG. 3(b) shows the optical signal generated by the transmitter. The “zero” level has shifted and is now at the level I0. Since the levels of the optical signals I0 and I1 are now hardly distinguishable, this creates possible errors in the information transfer.
Thus, there is a need in the art to develop a robust method of the information transfer which is equivalent to a multi-level encoding, but at the same time robust against temperature variations and corresponding deviations of the characteristics of the optoelectronic devices.